"The Content-Type metadata of the result of fetching a representation from a
resource depends on the URI scheme and corresponding protocol. However, there
are some circumstances where additional heuristics (overriding the protocol
defaults) are needed for compatibility with current web sites. Guidelines for
determining an appropriate content-type to presume are being developed
elsewhere [MIMESNIFF]; in particular, user agents MUST NOT use additional
heuristics or override authoritative metadata in ways that are not explicitly
allowed."